The ability to track the location or identity of a component or item of interest has presented a significant challenge for industry and science. For example, the demands of keeping track of consumer products, such as items in a grocery store or jewelry, and the interest in identification devices, such as security cards, has led to the need for a secure and convenient system. Additionally, emerging technologies such as combinatorial chemistry, genomics research, and microfluidics also require the ability to identify and track the location of large numbers of items.
A traditionally used method for tracking the location or identity of a component or item of interest is Universal Product Code technology, or barcode technology, which uses a linear array of elements that are either printed directly on an object or on labels that are affixed to the object. These bar code elements typically comprise bars and spaces, with bars of varying widths representing strings of binary ones and spaces of varying widths representing strings of binary zeros. Bar codes can be detectable optically using devices such as scanning laser beams or handheld wands, or they can be implemented in magnetic media. The readers and scanning systems electro-optically decode the symbol to multiple alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. Such characters are typically represented in digital form as an input to a data processing system for applications in point-of-sale processing and inventory control to name a few.
Although traditional bar codes typically only contain five or six letters or digits, two dimensional barcodes have also been developed in which one-dimensional bar codes are stacked with horizontal guard bars between them to increase the information density. For example, U.S. Pat. No. 5,304,786 describes the use of a high-density two-dimensional bar code symbol for use in bar code applications. Unfortunately, although the information density of barcode technology has improved, this technology is often easily destructible, and the interference of dust, dirt and physical damage limits the accuracy of the information acquired from the readout equipment. Additionally, because of the difficulty of etching the barcode on many items, it is also difficult to apply to a wide range of uses.
Another technology that has been developed for labeling objects includes a composition comprising silicon or silicon dioxide microparticles and a powder, fluid or gas to be applied to objects such as vehicles, credit cards and jewelry (WO 95/29437). This system typically allows the formation of 200 million particles on a single wafer, each of the particles on one wafer being designed to be of identical shape and size so that when the particles are freed from the wafer substrate one is left with a suspension containing a single particle type which can thus be identified and associated with a particular item of interest. This system, although information dense, is also not practical for a wide range of application. One of the advantages explicitly stated in the application includes the unlikely event of unauthorized replication of the particles because of the non-trivial process of micromachining used, which requires specialized equipment and skills. Thus, this process would not be widely amenable to a range of uses for inventory control.
In addition to above-mentioned barcoding and microparticle inventory control schemes, emerging technologies such as combinatorial chemistry have also resulted in the development of various encoding schemes (See, for example, Czarnik, A. W., “Encoding Methods for Combinatorial Chemistry”, Curr. Opin. Chem. Biol., 1997, 1, 60). The need for this development has arisen in part from the split and pool technique utilized in combinatorial chemistry to generate libraries on the order of one million compounds. Split and pool synthesis involves dividing a collection in beads into N groups, where N represents the number of different reagents being used in a particular reaction stage, and after the reaction is performed, pooling all of these groups together and repeating the split and pool process until the desired reaction sequence is completed. Clearly, in order to keep track of each of the compounds produced from a reaction series, the beads must be “tagged” or encoded with information at each stage to enable identification of the compound of interest or the reaction pathway producing the compound. The tags used to encode the information, however, must be robust to the conditions being employed in the chemical synthesis and must be easily identifiable to obtain the information. Exemplary encoding techniques that have been developed include the use of chemically robust small organic molecules (“tags”) that are cleaved from the bead after the synthesis is completed and analyzed using mass spectroscopy. (U.S. Pat. Nos. 5,565,324; 5,721,099). The disadvantage of this method is that the “tags” must be cleaved from the bead in order to gain information about the identity of the compound of interest.
In response to this, several groups have developed encoding schemes that allow analysis while the “tags” are still attached to the supports. For example, Radiofrequency Encoded Combinatorial (REC™) chemistry combines recent advances in microelectronics, sensors, and chemistry and uses a Single or Multiple Addressable Radiofrequency Tag (SMART™) semiconductor unit to record encoding and other relevant information along the synthetic pathway (Nicolaou et al., Angew. Chem. Int. Ed. Engl. 1995, 34, 2289). The disadvantage of this system, however, is that the SMART™ memory devices utilized are very large in size (mm), and thus scanning the bead to decode the information becomes difficult. Another example of on-bead decoding includes the use of colored and fluorescent beads (Egner et al., Chem. Commun. 1997, 735), in which a confocal microscope laser system was used to obtain the fluorescence spectra of fluorescent dyes. The drawback of this method, however, is the tendency of the dyes to undergo internal quenching by either energy transfer or reabsorption of the emitted light. Additionally, this system is not able to identify uniquely and distinctly a range of dyes.
Clearly, it would be desirable to develop a general information dense encoding system flexible, robust and practical enough to be utilized both in general inventory control and in emerging technologies. This system would also be capable of distinctly and uniquely identifying particular items or components of interest.